The general aim of INTERSALT is, jointly in London and Chicago, to coordinate an international study to test 3 hypotheses on the relationship between electrolytes and blood pressure. The specific aim is to undertake the final stages of this study (now almost 50% complete) including laboratory and data analyses and preparations of manuscripts on results. The 3 hypotheses are: a) there is a linear relationship between population mean sodium (Na) intake and average (median) blood pressure (BP) as well as prevalence of high blood pressure (HBP); b) there is an inverse association between mean population potassium (K) intake and BP-HBP; c) there is a positive correlation between population Na/K and BP-HBP. These hypotheses are being studied in 56 population samples worldwide with a wide range of Na intake. The first hypothesis, while current for years, has never been tested under a common protocol, with standardized measurement of BP, urinary electrolytes and possible confounding variables. INTERSALT is working to accomplish this is a systematic manner, following a joint protocol, selecting random samples from defined populations in each country, with 25 men and 25 women in each decade from 20-59 years (200 total), measuring BP by trained and certified observers using the Hawksley Random Zero Sphygmomanometer to minimize observer bias, collecting carefully timed 24-hr urine specimens, and completing a common questionnaire on possible confounding variables. Urine specimens are being analyzed for electrolytes and creatinine in the Central Laboratory at Leuven, Belgium. Centralized training has been completed via 5 regional meetings prior to start of field work, with site visits to selected centers. A detailed Manual Operations was prepared and served as the basis for the central training meetings. Additional teaching materials (slides, etc.) were also prepared for these meetings and then distributed (together with copies of the Manual) to all 56 centers for local training. Field work has been completed at this writing in 13 centers and specimens for 5,000+ individuals have been shipped to the Central Laboratory. Data forms are being coded, edited and entered into the computer. Field work is currently in progress in the Chicago sample. This application is a request for funding to assist completion of the central functions of the two Coordinating Centers (Chicago and London) and of the Central Laboratory. Rigorous testing of INTERSALT hypotheses has important implications for dietary approaches to the primary prevention and control of hypertension, a mass disease.